This invention relates generally to electrical circuits for operating arc discharge flashlamps and, more particularly, to a more efficient circuit for operating a plurality of flashlamps which are directly coupled to an alternating current (AC) source.
Such flashlamps are employed in a variety of applications; for example, flash photography; reprographic machines; laser excitation; and warning flashers for airplanes, towers, road barriers, marine equipment and tower mounted approach lighting systems for airport runways.
Flash lamps of the type referred to herein generally comprise two spaced apart electrodes within an hermetically sealed glass envelope having a rare gas fill, typically xenon, at a subatmospheric pressure. In typical prior art operating circuits, such lamps are connected across an energy storage device, such as one or more capacitors, charged to a substantial potential, but insufficient to ionize the xenon gas fill. Upon application of an additional pulse of sufficient voltage, the xenon is ionized and an electric arc is formed between the two electrodes, discharging the storage device through the flash lamp, which emits a burst of intense light. In many cases the pulse voltage is applied between an external trigger electrode, such as a wire wrapped around the envelope, and one of the electrodes; this is referred to as a shunt triggering. However, in other cases an external wire is not feasible since it may result in an undesirable arcing between the trigger wire and a proximate lamp reflector, or else the high potential applied to the external trigger wire might be hazardous to operating personnel. In those cases, the lamp may be internally triggered by applying the pulse voltage directly across the lamp electrodes, a technique referred to as injection triggering. Usually the voltage required is about 30 to 50 percent higher than that required to trigger the same lamp with an external trigger wire, and the trigger transformer secondary must carry the full lamp current.
In applications requiring two (or more) flash lamps, the lamps have been series-connected across the storage capacitor means, with a single injection trigger circuit being used for the series lamp combination. Whether using one lamp or a plurality of lamps, the general operation of the prior flash circuits comprised charging the storage capacitor means, typically through a resistor, to a predetermined level of voltage, then, on command, triggering the lamp (or lamps) into ionization and thereby discharging the capacitor means through the ionized lamp (or lamps). The energy thus developed in the lamp (or set of lamps) is equal to one-half of the capacitance of the storage means multiplied by the square of the charged voltage. Accordingly, this conventional method of operation results in the waste of a considerable amount of energy in charging the storage capacitor means through a power dissipating resistor. Further, time is wasted in "coming up to charge", or the storage capacitor means must be maintained in a fully charged state until called upon to flash the lamp.
One approach for overcoiming the aforementioned shortcomings of conventional flash lamp arrangements is described in the above-referenced copending applications Ser. No. 865,405 of Kirkhuff et al. Briefly, the operating circuit of this copending application uses the charging current of the storage capacitor, as well as the discharge current, for purposes of lamp energization. More specifically, first and second arc discharge flash lamps are series connected across a supply voltage source comprising a large direct current storage bank. The storage capacitor means is connected between the junction of the lamps and one terminal of the source. Respective injection or shunt means are provided for coupling trigger pulses to each lamp, and a succession of high voltage trigger pulses are alternately applied through the respective coupling means to the lamps. Each trigger pulse applied to the first lamp effects an arc path therethrough for charging the capacitor, and each trigger pulse applied to the second lamp effects an arc path therethrough for discharging the capacitor. Hence, the storage capacitor is charged through one lamp and discharged through the other in response to trigger pulses, which are applied in alternate sequence to the lamps. In essence, the lamps function as alternately actuated switches for charging and discharging the capacitor.
The flashes can be synchronized so that the human eye cannot perceive any variation in time between the flashes, e.g., four milliseconds between flashes. Such multiflash capability for predetermined durations is particularly useful for reprographic applications.
Efficiency is significantly increased by the elimination of power dissipating and time consuming charging resistors. The capacitor means delivers approximately twice the normal power to the lamp by virtue of its charging current as well as its discharge current. Accordingly, the capacitance for a given multiflash system in which the charge cycle is used for lamp energization, as well as the discharge cycle, may be approximately one half that required for the storage capacitor of a comparable system (i.e., same voltage and joule rating) employing a conventional resistor charge circuit. As a result, the circuit permits the use of a smaller capacitor with attendant reductions in cost and package size.
Further, the tendency of the arc discharge to hang on is reduced as each lamp functions as a switch, and the buildup of the voltage on the storage capacitor with respect to the source causes the first lamp (during the charge cycle) to extinguish at the proper time. During the discharge cycle, the second lamp extinguishes due to the limited energy capacity of the storage capacitor with respect to the source.
Although offering a number of significant advantages, the above-discussed circuit also has a disadvantage in that the power source requires a large DC storage means, such as a bank of capacitors. This tends to add to the bulk, weight and expense of the DC power source. Such factors detract from efforts to provide compact, low cost photographic flashlamps, or light weight runway flashers for mounting on frangible towers. One approach which has been taken to overcome such disadvantages with respect to the discharge storage bank (not power source) used in single flash lamp circuits is discussed to in the above-referenced copending application Ser. No. 775,122 of Kirkhuff et al, now U.S. Pat. No. 4,095,140. Briefly, the lamp is coupled directly across a conventional AC source to take advantage of the high transient current capacity thereof for flash operation. Triggering is controlled by an RC timing circuit at a predetermined phase of the AC source. This arrangement eliminates the charging resistor and discharge capacitor, but usually a series ballast resistor is required for current limiting, unless the lamp is optimized. Further, this direct line coupled system does not provide all the above-discussed advantages of the multiflash circuit which flashes lamps on both the charge and discharge cycles.